Electrical signals useful for data transmission and data computation propagate at high speed along electrical transmission lines that are constructed for low electrical capacitance, for low electrical attenuation, for shielding against cross talk and for slender physical size to permit dense grouping together as in high density wiring in electronic equipment.
A typical electrical cable suitable for use as a transmission line for propogation of high speed electrical signals is comprised of a conductive slender wire serving as a signal carrying conductor surrounded concentrically by a solid dielectric material with a dielectric constant carefully selected for minimized impedance to the velocity of the signals, in turn surrounded by a conductive foil that is conductively joined with a slender conductive drain wire that extends the length of the foil and serves as a convenient electrical terminal to which an electrical connection can be made and established as an electrical connection with the foil.
The cable is carefully constructed with a geometry of small physical size and a constant electrical characteristic impedance. The small physical size of the cable must be appreciated. For example, a cable of 50 Ohms characteristic impedance has a signal carrying conductor size of 30 AWG, surrounded concentrically by a dielectric of 0.68 millimeter diameter, a drain wire size of 30 AWG sheathed in a dielectric jacket that provides the outer diameter of the cable of 1.78 millimeter maximum. The centerlines of the conductor and drain wire are spaced apart 0.49 millimeter.
A cable of 0.75 Ohms characteristic impedance has a signal carrying conductor size of 32 AWG, surrounded concentrically by a dielectric of 0.75 millimeter diameter, a drain wire size of 32 AWG sheathed in a dielectric jacket that provides the outer diameter of the cable of 1.78 millimeter maximum. The centerlines of the conductor and drain wire are spaced apart 0.50 millimeter.
A cable of 90 Ohms characteristic impedance has a signal carrying conductor size of 32 AWG, surrounded concentrically by a dielectric of 1.0 millimeter diameter, a drain wire size of 32 AWG sheathed in a dielectric jacket that provides the outer diameter of the cable of 1.78 millimeter maximum. The centerlines of the conductor and drain wire are spaced apart 0.73 millimeter.
The physical presence of an electrical connector assembly along an electrical cable for transmission of high speed electrical signals is a disruption of the carefully selected geometry of the cable. The connector assembly is a potential source of distortion of the signals. For example, the connector assembly is a potential source of signal cross talk in a backward or reverse direction of signal propogation. The connector assembly is a potential source of spurious signals caused by reflection of the propagating signals, especially propagating signals having voltage wave forms with short rise times.
An electrical connector assembly suitable for combination with a cable for transmission of high speed signals must be of small physical size to permit passage of propagating signals along the connector assembly with negligible cross talk and with minimum signal reflection. It is foreseen that a connector assembly of small size is difficult to manipulate during assembly with slender wires of a cable having the above-identified small dimensions. Despite its small size, the connector assembly must possess a geometry that is constructed for tactile identification and manual or machine manipulation, especially during assembly with a cable having slender wires.
Further, it is foreseen that a connector assembly of small size must separate the slender wires of the cable from one another to prevent an electrical short circuit. The connector assembly must establish electrical connections of the wires with corresponding electrical terminals of small metal parts in the connector assembly.
Accordingly, the connector assembly must be constructed for accurate manipulation of conductive slender wires of an electrical cable used as a transmission line for high speed electrical signals, and for accurate manipulation of small metal parts to form electrical connections between the slender wires and the small metal parts for use in an electrical connector assembly that causes negligible backward cross talk and minimum reflection of signals propagating in the transmission lines.
Inexorably, speeds of intelligence manipulation and transmission appear to go up. As this occurs, speeds of signal transmission follow suit. From the days of Morse Code, when bit rates could be measured in 50 characters per minute, the time has come when binary signals are regularly transmitted within a computer at rates exceeding millions of instructions per second. Indeed, there is a current need for intelligence to be transmitted via cables externally of computers to peripheral apparatus such as printers at speeds exceeding 10 to 12 kilohertz. With this progress, technology has moved from a time when a couple of wires hung on insulators carried on wooden poles were more than adequate, to a time when twisted pair cable is no longer sufficient for many requirements and the energy of intelligence itself is no longer carried essentially in the conductive material of the wire. This is because at the high speeds of transmission, the energy is propagated essentially in the dielectric medium proximate to the conductor of the wire and is of a nature easily interfered with by the electrical and the electromagnetic fields extant. With this change, concepts of impedance matching and the effects of physical dimensions of connector and cable have become critical.
As this has occurred, the need to design to accommodate higher signal speeds or the equivalent shorter signal wavelengths has become evident. Not obviously, but clearly, that need has manifested itself in design parameters calling for physically, dimensionally, smaller parts and portions of connectors and cable. This has been caused by the discovery that when such parts are physically very short compared to the wavelength or the equivalent pulse shape of the signals of transmission, there can be less signal reflection and a more efficient transmission through the cable and through associated connectors.
Hand-in-hand with the foregoing has been a reduction in size in both cable and connectors to a point wherein the ability to physically handle the constituent parts of both has proven difficult. When the wires of the cable are only several times in diameter of that of the human hair and the parts of the connector are necessarily smaller than a grain of rice and even the best of vision can no longer discern left from right in these elements, one can readily perceive the problems caused. Quite unfortunately, the problems which are manifest in the ability of human beings to do what is necessary to put connectors together and assemble connectors to cable are not necessarily solved merely by turning to machines to put things together. This is particularly so when the things that must be put together cannot be produced or reproduced by processes intrinsically capable of down scaling such as by photolithography, silk screening, or the like. Indeed when things must be made in three dimensions rather than two, miniaturization in manufacture and handling becomes quite difficult and operates as a barrier to progress.
With the foregoing as context, it is an object of the present invention to provide a method and apparatus for handling and terminating the small parts of signal transmission cable and related connectors in a manner which is practical in terms of manual assembly and automation.
It is a more specific object of the present invention to provide a method and apparatus which makes it easier to handle the parts and portions of connectors and cable which are of small physical dimension necessitated by the requirements of the signals transmitted thereby.
It is yet another object to provide a coaxial connector and technique for application thereof to cable which is both amenable to physical handling by human beings and as well by machines.
Finally, it is an object of the invention to provide an interconnection system of improved performance in the face of rigorous demands upon dimensional integrity.